The present invention relates generally to radar and communications systems operating on a sampling basis to detect targets in discrete range samples. In particular, it relates to means for improving detection by reducing the so-called `range-smearing` due to relative movement of the target.
In radar and communications systems it often is necessary to increase the signal level relative to the noise level by combining many pulse returns before a decision can be made as to the presence of a target signal or of a communications signal. With regard to radar systems, there is a further need of estimating the target size once it has been decided that a target signal is present in the radar case. As to communication systems, there is a similar need arising after detection to decide which of several possible signals is present. Many of these systems are `sampled systems` to the extent that the signals are not available in continuous form but only as discrete range samples. For example, a radar system transmits pulses at predetermined interpulse periods T and a range sample is received for each of the pulse transmissions. Each range sample represents a particular intersample range which can be identified by the letter R.
Many of these systems are complicated by situations in which there is a relative motion between, for example, the target and the radar case, or between the transmitting and receiving antennaes of a communication system. If this relative motion is large or if the pulse-to-pulse integration time is long, a range smearing results. This smearing, as is known, decreases the signal level so as to possibly obscure the signal from detection or, if detected, to materially reduce the ability to estimate the target size. A similar problem occurs in communication systems to the extent that the communication signal becomes more difficult to detect.
The present invention primarily is concerned with providing an effective and relatively simple method and apparatus for reducing the undesirable range smearing and this objective is achieved by integrating closest range samples in a manner adaptive to target motion. The particular manner in which the closest range sample integration is achieved, of course, will be described. In general, a succession of integrated range samples is stored in a shiftable storage register which then is shifted to assure that the closest stored range sample is integrated with each new incoming range sample. The shifting control is provided by a control circuit which estimates on a pulse-to-pulse basis the relative speed of the target. The estimate tells the shiftable register how many range sample shifts are needed to align the new incoming range sample with a previous integrated sum present in the shiftable register so that the register can shift to align that sum for integration with the new incoming signal.